Such lifting units have become known, for example, from U.S. Pat. Nos. 3,724,511, 3,726,323 and German Auslegeschrift No. 29 38 451 (corresponds to U.S. Pat. No. 4,354,531) . A characteristic of these known embodiments is that when the shifting wedge is not in engagement with the groove of the drive shaft, it is held by a locking mechanism which consists of a V-shaped, stationary notch or groove with springy side surfaces, which side surfaces are formed by special slide members which are under a spring force or by the ends of two approximately semicircular, resiliently mounted guide rails. The springy arrangement, which was chosen among others for safety reasons, prevents for example faulty control of the shifting wedge and/or the destruction of a machine part when a shifting wedge incompletely engages the groove of the drive shaft. Through an elastic yielding and springing back of the lateral notch boundaries, the outer end of the shifting wedge is fed to the notch or the groove.
In order to ensure that, during the next control of the shifting wedge, namely the disengagement of its outer end from the notch and the engagement of its inner end in a groove of the temporarily stationary drive shaft, the wedge does not hit the rigid groove edge of the shaft groove but instead slides interference-free into the groove, a precise maintaining of the specified position of the notch or groove and thus the shifting wedge is absolutely necessary.
In the known individually springily arranged side surfaces of the V-notch or groove, there is a possibility that one edge of the notch, which edge defines the side surface, will be urged back slightly due to an incorrect position of the wedge. The wedge is thus no longer exactly aligned with the shaft groove by the notch, so that it cannot move directly and unhindered into the groove of the drive shaft, which can result in a faulty control of the weaving machine.